An aerodynamic lens can be defined as a flow configuration in which a stream traveling through a cylindrical channel with diameter D is passed through an orifice with a smaller diameter d, undergoing one contraction upstream of the orifice and one subsequent and immediate expansion downstream of the orifice. A contraction of an aerosol stream is produced as the flow approaches and passes through the orifice. The gas then undergoes an expansion as the flow propagates downstream into a wider cross sectional area. Flow through the orifice forces particles towards the flow axis, so that the aerosol stream is narrowed and collimated. In the typical aerodynamic lens system, an aerosol stream is tightly confined around the axis of a flow cell by passing the particle distribution through a series of axisymmetric contractions and expansions. Each section of the lens system consisting of a flow channel and an orifice is defined as a stage. Aerosol streams collimated by an aerodynamic lens system have been designed for use in many fields, including pharmaceutical aerosol delivery and additive manufacturing.
Numerous studies have been performed to characterize the focusing effect created by propagating an aerosol stream through a single orifice consisting of a capillary tube, a converging nozzle, or a sheathed nozzle. See Cheng and Dahneke, J. Aerosol Sci. 10, 363 (1979); Dahneke, Nozzle-Inlet Design For Aerosol Beam Instruments, in J. J. Potter, Rarefied Gas Dynamics, Vol. II (pp. 1163-1172). New York: AIAA (1977); Dahneke, Aerosol Beams. In D. T. Shaw, Recent Developments in Aerosol Science (pp. 187-223). New York: John Wiley & Sons (1978); Dahneke, J. Aerosol Sci. 10, 257 (1979); Mallina, J. Aerosol Sci. 30, 719 (1999); and Mallina, Aerosol Sci. Technol. 33, 87 (2000). These theoretical and experimental studies conclude that single-orifice systems can only focus a narrow range of particle sizes to a sharp point. See R. Deng, Aerosol Science and Technology 42(11), 899 (2008). Specifically, single-orifice systems can focus a mono-dispersed aerosol distribution to a well-defined point, but poly-dispersed distributions will be focused at different positions along the flow axis, with the focal position and focused diameter dependent on the droplet size. A mathematical description of focusing of an aerosol stream passing through an orifice has been developed in terms of a critical Stokes number S*. See De la Mora, J. Fluid Mech. 195, 1 (1988). Particles with Stokes number above S*cross the flow axis at some finite distance from the lens, sub-critical particles do not cross the axis, and critical particles cross the axis at infinity.
In U.S. Pat. No. 4,019,188, Hochberg discloses an apparatus for producing a narrow, collimated stream of aerosol particles using a carrier gas jet and a surrounding sheath flow. The Hochberg apparatus uses a carrier gas velocity that forces particles to the center of the gas flow, surrounds the flow with a sheath gas, and directs the combined flow through a nozzle. A sheath flow upstream of an exit nozzle prevents impaction of particles onto the nozzle.
Many researchers have reported studies of aerodynamic focusing of aerosol streams using fixed multi-stage lens systems. Focusing of a stream of aerosol particles using a system of aerodynamic lenses was first reported by Lui in 1995. The system of Lui was used to narrow and collimate a beam of spherical particles with diameters of approximately 25 to 250 nanometers. Lui used a lens system having three to five stages, with emphasis placed on achieving a low pressure drop across each lens. Numerous experimental and theoretical studies have been performed subsequent to the work of Lui, considering the aerodynamic effects of single- and multi-orifice lens configurations.
In U.S. Pat. No. 6,348,687, Brockmann discloses an apparatus for generating a collimated aerosol beam of particles with diameters from 1 to 100 microns. Brockmann describes a multi-stage lens system that focuses large, solid particles. The aerodynamic lens system of Brockmann uses a series of fixed lens and an annular sheath gas surrounding a particle-laden carrier gas. The Brockmann apparatus also uses an annularly flowing sheath gas to prevent impaction of particles onto the orifice surfaces. The system of Brockmann was used to focus 15 micron aluminum particles to a diameter of 800 microns, and generally uses the same aerosol and sheath gas flow rates.
In U.S. Pat. No. 7,652,247, Lee discloses an aerodynamic lens system for focusing nanoparticles in air with diameters between 5 and 50 nanometers. In U.S. Pat. No. 8,119,977, Lee discloses a multi-stage, multi-orifice aerodynamic lens for focusing a range of particle diameters covering two orders of magnitude, from 30 to 3000 nanometers.
Aerodynamic focusing using a sheath gas is generally accomplished by propagating an annular sheath/aerosol flow through a continuously converging nozzle, using differing sheath and aerosol gas flow rates. The degree of focusing is proportional to the ratio of the gas flows. In U.S. Pat. No. 7,108,894, Renn discloses a method of aerodynamic focusing using a coaxial sheath gas flow that surrounds an aerosol-laden carrier gas. The combined flow is then propagated through a converging nozzle. Renn teaches that for the operational range of a flow system using a sheathed aerosol stream and a single converging nozzle, the diameter of the focused beam is a strong function of the ratio of the sheath to aerosol gas flow rates.
Various systems using aerosol-based or droplet-based deposition techniques have been reported as viable methods for direct printing or additive manufacturing. In U.S. Pat. No. 6,924,004, Rao discloses a method and apparatus for depositing films and coatings from a nanoparticle stream focused using an aerodynamic lens system. The apparatus of Rao uses high-speed impaction to deposit nanoparticles on a substrate. A method of separating particles from a gas flow using successive expansions and compressions of the flow created by an aerodynamic lens is discussed by Novosselov in U.S. Pat. No. 8,561,486.
In all cases, the material stream arriving at the substrate must be temporarily interrupted to allow for deposition of discreet structures. For systems wherein particles are produced and transported in a continuous mode, the stream can be interrupted using impact or non-impact methods. In on-demand systems, a single particle is produced by the application of a suitable force to a volume of fluid situated above an orifice. In some on-demand systems, it is preferable to provide a constant production of particles, while shuttering the resulting stream. The on-demand process can use impact or non-impact shuttering.
Impact shuttering schemes place a solid blade or spoon-like shutter in the aerosol stream, so that particles maintain the original flow direction but are collected on the shutter surface. Impact shutters typically use an electromechanical configuration wherein a voltage pulse is applied to a solenoid that rotates the shutter into the path of the aerosol stream. Impact-based shuttering can cause defocusing of the particle stream as the shutter passes through the aerosol stream. Impact shutters can also cause extraneous particle deposition as excess material accumulates on the shutter surface.
Aerosol shuttering can also use a pneumatic shutter to divert the aerosol stream from the original flow direction and into a collection chamber or to an exhaust port. Pneumatic shuttering is a non-impact process, so that no aerosol accumulation occurs above the substrate. Non-impact shuttering schemes can have shutter on/off times as small as 10 milliseconds.
External shuttering apparatuses use a shuttering mechanism that is external to the flow cell nozzle, and typically increase the working distance of the deposition head. An increased working distance can lead to deposition at a non-optimal nozzle to substrate distance. With internal shuttering, shuttering occurs in the interior of the flow cell, upstream of the final flow orifice. Internal shutters allow for the minimal nozzle to substrate distance, so that optimal focusing or collimation of the aerosol stream is provided.